With recently increased environmental concern, many researches regarding an electric vehicle or a hybrid electric vehicle that can substitute a vehicle using fossil fuel such as a gasoline vehicle or a diesel vehicle which is one of main causes of air pollution, are conducted. As a power source for the electric vehicle and the hybrid electric vehicle, a nickel hydrogen metal secondary battery is mainly used. However, researches for using a lithium secondary battery having high energy density and discharge voltage are actively conducted, and some of these are in the stages of commercialization.
For a negative electrode active material having high input and output characteristics, lithium titanium oxide (LTO) is gaining increasing attention. Spinel lithium titanium oxide, which is a representative example of the oxide having lithium intercalation and de-intercalation at a state of maintaining crystal structure, has been first introduced in 1971. Spinel lithium titanium oxide raised great interest for its possibility to be an excellent electrode material or mass storage material, because it has a superb mobility of lithium ions, and the material structure is not changed during charging and discharging. A lithium secondary battery including such lithium titanium oxide as a negative electrode active material seldom has electrolyte disintegration because oxidative/reductive potential of the negative electrode is about 1.5 V with respect to Li/Li+ potential, which is relatively high, and also has excellent cycle characteristics because of stability of a crystal structure. However, lithium titanium oxide has disadvantages in which capacity per unit weight is small and energy density is low.
In consideration of this, related technology suggests a negative electrode material including a carbon-based material and lithium titanium oxide. For example, Japanese Patent Publication No. 1998-069922 discloses a negative electrode added with lithium titanium composite oxide as a main active material and an active material having low oxidative/reductive potential as a sub-active material. Further, Japanese Patent Publication No. 2006-278282 discloses a technology of adding titanic acid lithium of a spinel structure as the negative electrode active material and a carbonaceous material as a conductor. However, the negative electrode materials using lithium titanium oxide as a main active material do not solve the problems concerning little capacity and low energy density of lithium titanium-based oxide. Accordingly, there is a high demand for a negative electrode material that can make up for the shortcomings of lithium titanium oxide, and that has a low internal resistance and high electrical conductivity, and improved output characteristics.